Flew on the A220 for the first time this weekend. What is with the god awful noises with this plane while taxiing? If anybody wants to experience them go to seat 21a. Once airborne it is fine. I can only assume it is a combination of the brakes and maybe a fuel pump? It sounds like servos grinding away until we got rolling.
Welcome to the (bizarre) Bombardier hydraulic philosophy of running a standby pump AND a PTU (Power Transfer Unit) ALL THE TIME, until you're in the air with gear and flaps up.
It's like the barking dog noise that people hear on the A320, except on the A320 it typically stops once the second engine spools up.
I have no idea why they designed the CSeries/A220 this way; it drives me nuts. I hate listening to that darn thing.
Over in Boeing 737 land, we simply run TWO engine-driven pumps and TWO electric motor pumps all the time. And they're nowhere near as loud as a PTU. Plus, with four pumps, you've got a lot more leeway for deferrals on the 737.
I love the A220, but there are already several things about it that I'm not real crazy about.